1. Field of the Invention
The present invention relates to a switch apparatus with a membrane switch which is useful as a control switch of an electronic device or the like.
2. Description of the Prior Art
A membrane switch is a switch in which a resin panel is disposed on a substrate via a spacer having an opening which is in an operating portion. When the operating portion of the resin panel is pressed, conducting members disposed in the operating portion are short-circuited.
One example of such a membrane switch is shown in FIG. 14. In this example, flat patterns 3 and 4 which are electrically conductive are formed on the upper surface of a substrate 1 and on the lower surface of a resin panel 2, respectively. When the resin panel 2 is pressed from above, the conductive patterns 3 and 4 which have been separated from each other by a spacer 5 come into contact with each other to be short-circuited, whereby the switch turns ON.
Another example of a conventional membrane switch is shown in FIGS. 15 and 16. In this example, two comb-shaped conductive patterns 3 and 4 are formed on the upper surface of a substrate 1, and an electrically conductive pattern 6 made of conductive rubber is stuck on the lower surface of a resin panel 2. When the resin panel 2 is pressed from above, the conductive pattern 6 comes into contact with the conductive patterns 3 and 4 to short-circuit the two patterns, whereby the switch turns ON.
Membrane switches having the above-mentioned structure, however, are disadvantageous in that the conductors functioning as contact points easily peel off, in particular, the conductive pattern 4 and conductive pattern 6 on the resin panel 2 are prone to peel off because the resin panel 2 is deformed by the pressing operation. The short-circuited state due to the peeling is difficult to distinguish from the short-circuited state in the normal switch operation, because these short-circuited states are mostly unstable. When a break occurs between the conductor and a lead due to the peeling of the conductor, it is impossible to distinguish this breaking state from the non-operation state. In conventional membrane switches, failures due to the peeling of conductors easily happen, and cannot be easily detected. Therefore, conventional membrane switches have a problem in that they cannot be used for devices which are required to have high reliability.
When the conductors are short-circuited as a result of the peeling of one of the conductors, the switch turns ON without attracting anyone's attention. Therefore, some switches are provided with a function that the period of time for which the switch continues to be in the ON state is monitored and if the ON state continues for a predetermined period of time (e.g. 10 seconds) or more, the ON state is judged not to be a normal one caused by the operation of a person, thereby sending an alarm signal. A conventional switch apparatus having such a function will be described with reference to FIGS. 17 and 18.
In this switch apparatus, a conductive pattern 3 is connected to a constant voltage source through a resistor 21, and a conductive pattern 4 is grounded. The connecting point 22 between the conductive pattern 3 and the resistor 21 is connected to a microcomputer 23. The microcomputer 23 detects the potential of the connecting point 22 to judge whether the switch is in the ON state or not. FIG. 18 shows a flow of the switch state detecting routine in the microcomputer 23. The switch state detecting routine is called every 100 milliseconds by a timer interruption. First, it is examined whether the conductive patterns 3 and 4 are electrically connected or not, in other words, whether the potential of the connecting point 22 is low or not (step S21). If the potential of the connecting point 22 is not low, a counter in the microcomputer 23 is reset (step S26), and the process returns to the main routine. If the potential of the connecting point 22 is low at step S21, the counter is incremented by 1 (step S22). In step S23, it is examined whether the contents of the counter exceed 100 or not (in other words, whether the potential of the connecting point 22 is continuously low for 10 seconds or longer or not). In normal operation, the switch will not be operated continuously for 10 seconds or more, so that the process proceeds to step S24. In step S24, it is examined whether the contents of the counter are equal to 2 or not. If the contents are not equal to 2, the process returns to the main routine. If the contents are equal to 2, the process proceeds to step S25, at which the switch ON process is performed. If the contents of the counter exceed 100 at step S23 (in other words, if the potential of the connecting point 22 is continuously low for 10 seconds or more), it is judged that a failure has occurred, and an alarm signal is dispatched. According to the above-described process, in the second switch state detecting routine after the switch operation (i.e., when the ON operation continues for about 0.2 seconds or more), the computer performs the switch ON process. When the ON state continues for 10 seconds or longer, an alarm for switch failure is given.
In some of the short-circuit failures due to the peeling of a conductor in a switch having the membrane structure, the switch turns ON only when being vibrated because of the unstable contact between the conductors. Such a short-circuit in a short time can not be detected by the above-mentioned failure detection. Accordingly, the failure detection is not sufficiently performed in a prior art switch apparatus.